1. Field of the Invention
The present invention relates to a focus detection apparatus. More particularly, the invention relates to a focus detection apparatus suitable for a photographic camera, video camera, or the like in which the imaging state of an objective image formed by a photographing lens is detected to perform the focus detection for the photographing lens.
2. Related Background Art
Traditionally, there have been known various focus detection apparatuses for a single lens reflex camera and the like. For example, a method for determining a focusing state by metering several places in a photographing image plane is disclosed in Japanese Patent Laid-Open Application No. 63-13010 and Japanese Patent Laid-Open Application No. 1-120520. The focus direction apparatus of a type represented by these publications is such that a part of a flap mirror which guides the rays of light from an object to the finder system is structured with a semitransparent film to utilize the transmitting light through such a part of the mirror for the focus detection. The system is arranged so that the transmitted light is reflected in the lower direction to the camera body through a sub-mirror provided behind the flap mirror to be received by the detection unit which is arranged at the bottom of the mirror box. A disadvantage of this method is that there is a limitation on the size of the sub-mirror due to the arrangement required so as not to cause the imaging light beam to be eclipsed at the time of photographing. Particularly, it is difficult for the system to arrange metering fields in the upper and lower regions of the image plane to be photographed.
A focus detection apparatus disclosed in Japanese Patent Laid-Open Application No. 2-24616 is designed to eliminiate the limitation described above. In such an apparatus, the limitation caused by the sub-mirror is eliminated by a partially shared use of the finder optical system and focus detection optical system for a single lens reflex camera. As a result, it possible to arrange a layout of the metering field in a wide area of the photographing image plane.
Also, in Japanese Patent Laid-Open Application No. 64-44906, a detailed description is made of the relationship between the focus detection light beam and the effective F number, F No., which is a common restraint on the phase difference detection method adopted as the focus detection method disclosed in the aforesaid publication. In this application, there is a disclosure of a technique which causes the computation of a focus detection to be disabled if the focus detection beam is eclipsed.
As has been described, in a single lens reflex camera and like apparatuses, a focus detection method, which is free from any restraint on the photographing image plane, is in demand. However, there are several restrictive items which should be cleared before implementing such a detection method practically.
A first item is that the quality of the optical image, which is formed actually on a photoelectric conversion element for performing the phase difference detection, should be a desirable one. When a detection is actually performed by an image sensor or the like, the fine pattern of an object cannot be resolved on the photoelectric conversion element in some cases if its dotted image is not evenly formed depending on the position on the image plane. Now, as these fine patterns have the most information regarding the required focus adjustment, the inferior resolution causes the degradation of the accurate performance of the focus detection. Also, the degree of correlation between the two images detected as a pair is lowered. As a result, the accuracy of the focus detection is degraded.
A second item is the problem of cost as a matter of course.
If the aforesaid conventional examples are reviewed from the lost viewpoint the structure disclosed in the Japanese Patent Laid-Open Application No. 2-24616, for example, uses only one piece of biconvex lens for the reimaging optical system, which is simply positioned behind its iris. Therefore, although there is a disclosure of a structure in which the light having a high angle of view can be drawn while avoiding the restraint on the width of light beam due to the sub-mirror, its imaging performance in the circumference of the image plane is not desirable. This is against the first restrictive items mentioned above.
In order to improve the imaging performance, it may be one of the solutions to adopt a lens structure such as a triplet for the reimaging optical system, but this results in a significant problem of a higher cost, and is against the aforesaid second item. This is far from being realistic.
Also, in the aforesaid Japanese Patent Laid-Open Application No. 63-1310 and Japanese Patent Laid-Open Application No. 1-120520, a technique is disclosed to divide a condenser lens into a plurality of areas to optimize it for each of the metering fields. However, in the objective method by the application hereof, the use of the condenser lens is shared with the finder optical system, and the disclosed technique can hardly be adopted. Eventually, therefore, it is difficult to provide the metering field in the wide area of the photographing image plane.
With the structure of the optical system in view, the common use of an optical path for both a focus detection system and finder optical system presents another problem in improving the imaging performance referred to in the aforesaid first item. Such a problem is due to a pentagonal prism which is incorporated in the finder system. The common use of the pentagonal prism necessitates making the optical path long for the optical system which causes the photoelectric conversion element to perform reimaging for the focus detection. As compared with the conventional type such as storing a detection system at the bottom of a mirror box, the optical path is several times longer. Supposing that the length of pixel array of a photoelectric conversion element to be used is defined the same as the conventional one, it is necessary to make the imaging magnification equal for the optical system as a whole even if the length of the optical path becomes longer. Then, an optical system having a desired imaging relationship is obtainable if the reimaging lens is enlarged in analogue by a magnification equal to the portion of the optical path which has become longer than its original length. However, the application of a proportional enlargement such as this results in the enlargement of the aberration values with respect to the length after all. On the other hand, however, the allowable value of aberrations for the system as a whole is invariable. Therefore, such a corrective measure as attempted by a simple enlargement brings about a contradiction. In fact, an aberration such as a spherical aberration, coma aberration, and chromatic aberration is deteriorated more by a given magnification. Particularly, the deterioration of the spherical aberration causes a dotted image to be widened, leading to an inferior resolution of the fine pattern of an object. Accordingly, the fine pattern detection performance is degraded to cause the focus detection capability to be extremely lowered. For the reimaging optical system which needs a longer optical path as aimed at by the present invention, it is necessary to make the dotted images as small as possible as its prime design consideration.